LGS WFS parameters
3
LGS HOWFS
(9)
after WF
relay589 nm 727
16x16, 32x32, 64x64
1.45no
filter
64x64, 32x32, 16x16
174Theta-phi
OSM
Location Type Range of travelMin. step size
(est.)LGS channel
(individual unit focus)
delta Z ~10 mm 10 um
LGS lenslet changerdeltaX,
Ydelta X = 20 mm, delta
Y = 2 mm1 um
LGS channel relay and camera focus for pupil sampling
change
delta Z 5 mm 0.1 um
Pick off (theta mech)delta theta
360 deg.0.2 (UP)/ 115"
(LA)
Pick off (phi mech.)delta phi
360 deg.0.2" (UP)/ 300"
(LA)LGS WFS unit overall
traveldelta Z 130 mm 1 mm
Rotation of the WFS (sans the central
WFS)
delta (theta-
phi)360 deg. 100"
WFS type Length in Z [mm]
Breadth X [mm]
Width Y [mm]
# of sensors Pick off mech. Cooling
LGS WFS 850 150 150 9 LGS theta-phi 3 stage Peltier
WFS type (# of sensors)
LocationSensing
wavelength (nm)
Input platesc
ale (um/")
# of sub-apertures
Detector platescale("
/pixel)Filters
Pupil sampling
Field of regard
(arcsec)Comments
Post SF review changes to LGS WFS architecture
• 4 fixed + 3 (PnS) LGS sensors (as compared to 1 fixed + 8 mobile sensors)
• Field of regard of PnS sensors is 120 arcsec dia. and the fixed sensors are positioned to accept LGS light at 10 arcsec. radius (as compared to 10-150 arcsec continuous FoR (dia.) for sensors in the pentagon formation and 10-174” arcsec continuous FoR (dia.) for PnS sensors)
• Only 2 pupil sampling scales – 64x64, 32x32 (no 16x16).
• LGS TT mirror to be implemented on the downlink – not appraised as part of WFS costs, neither have the motion controls for these mirrors defined as part of LGS WFS (this was a recent change).
• LGS WFS’s are not refrigerated and are fed through a window (or a pair of windows) – sensors are not required to operate -17C but need to work at -5 C.
• Lesser motion control
Total laser power has been reduced and the sodium return numbers have been scaled accordingly for performance in Mauna Kea.
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LGS WFS design concept (needs refinement)
7
Fixed frame
Camera Rotation axis
Camera Translation4 Fixed Cameras
Reflective tetrahedron
LGS WFS TT stages are not implemented yet (we realized the need to accommodate TT sensor last week).
LGS WFS issues• The most extreme RMS wavefront error in the LGS path is 1.5-1.7micron, and the
largest spot size is about 250 miliarcsec - these static aberrations on the LGS caused a shift of the centroids of 5% of a sub-aperture (at most) [Kupke]
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Laser Guide Star Size Calculation
Finite Object Size
Intrinsic guide star diameter 0.00 arcsec
Uplink formation of the beacon(s)
Perfect Uplink AO? NO
Inherent aberrations in the uplink beam: 0.90 arcsec
Beam movement contribution to uplink 0.33 arcsec
Residual seeing contribution to uplink 0.60 arcsec
Diameter of point source laser at Na layer: 1.08 arcsec
Seeing
Natural seeing FWHM at GS wavelength 0.58 arcsec
Subaperture Tip/Tilt corrected FWHM 0.47 arcsec
Contribution due to seeing 0.47 arcsec
Elongation
Distance from LLT to telescope axis: 0.00 m
Use Max. Elongation? NO 1.27 arcsec
Avg. Elongation 0.85 arcsec
Contribution due to elongation 0.85 arcsec
System Aberrations
Aberrations in AO thru to WFS 0.25 arcsec
Atmospheric Dispersion
ADC in HOWFS? NO
RMS blurring due to atmospheric dispersion 0.000 arcsec
Total size of detected return beam: 1.47 arcsec
Sensing Approach
Pyramid WFS? NO
Charge Diffusion
Charge Diffusion 0.25 pixels
Contribution due to Charge Diffusion 0.40 arcsec
Subaperture Diffraction
Lambda/d (for sensing) 0.66 arcsec
Spot size used for centroiding 1.66 arcsec
We have started working with Lick to understand what WF quality is required at the LGS input (Gemini and TMT may have over-specified their sensor).
LGS WFS design concept
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Pick off arms search
area
Pick off arms effective 120”
search area
Tetrahedron to direct light to fixed WFS’s.
Issues & to-dos:
• Need to update all relevant requirements and nail down as many #s and parameters as possible.
• Refine the LGS sensor concept to work as 4 fixed + 3 PnS (conceptual work has already started).
• Redesign pick off arm with pupil access and TTM.• Now that a 256x256 pixel CCD is being talked about in meetings –
should we make it our baseline? If so, what is the pixel size? • We need to model 2 more windows in zemax as the sensor resides
outside the refrigerated chamber.
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LGS spots with sensor in the current optical design
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The AO design team and the WFS team are trying to hash out details to be able to build a integrated optical model with version control.
Other WFS related changes
• No narrow field TWFS.• No ADC on NGS WFS.• No ADC in the TWFS.
• In general, the system has undergone many changes, any design work without nailing down requirements might mean re-work.
• Next 2 weeks will mostly be spent in updating the requirements database.
• Alex is already obtained a Contour account and is getting familiar with the software, the requirements and the PBS.
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